@david - I believe standard accepted practice now is NOT to buddy up to treat hypothermia. Best to construct a 1-person burrito wrap. Should be its own question on here if not already.
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LBellFeb 2 '12 at 2:15

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Lots of good thoughts - but one thing I know is true: if you are cold and want to warm up, try taking off (and putting back on) all of your clothes without getting out of your sleeping bag. Works every time.
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LBellNov 15 '12 at 14:19

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This seems to have attracted a lot of contradictory answers, none of them containing any solid evidence. This question might be better for skeptics.SE.
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Ben CrowellMar 4 '14 at 4:15

11 Answers
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When talking about fresh, dry clothes then it's not true. More layers definitely equals warmer!

As pointed out in the comments if you really go to extremes then more layers doesn't necessarily equal warmer, but to get to that point you have to really cram yourself in the bag so there's no insulating air between the layers. You could also make yourself so warm you sweat which would make your clothes damp and cool you down again, but if you just wear warm enough clothes to be comfortable, not sweltering (which is surely what you want anyway!) then you shouldn't have an issue.

Where this saying may come into effect (I've heard it branded around before as well) is when you wear clothes to bed that you've already been wearing so they may be damp, or clothes that are next to damp clothes in your rucksack (somewhat surprisingly regular for the more novice hikers!) and so on with similar situations. In those cases, it's probably best to sleep without than put on damp clothes.

Yes, in certain contrived / specialised / odd cases this may be true. But on all practical levels my experiences, and the experiences of those I know suggest that it's a myth with dry clothes and only comes into play with damp clothes, which most seasoned campers (and many other people too) know tend to easily cool you down anyway!

Possibly. the reason this is a consideration is best way to stay warm is with loose layers (multiple depending on the temp) that trap air pockets close to the body that are heated BY the body. if you are in your birthday suit you will trap a decent larger pocket of air around you.

BUT a single sleeping bag will NOT keep you warm this way. if you go this route here are a few things to keep in mind

SWEAT is your enemy. I personally would NOT go completely naked but have 1 or 2 layers on. the first would be a wicking layer like under-armor or a polypropylene or silk layer (NOT COTTON)

Being that your don't have as many layers ON you. you will need to re-locate those layers to the other side of the sleeping bag. that may be other sleeping bags, a blanket, a tarp around that, or (most likely a combination there of)

once you get out of your sleeping bag it will get REAL cold REAL quick. therefore I would leave my layers for the next day INSIDE my sleeping bag to warm up with me.

four words. "Bathroom trip ain't fun." you may want to keep a "pee jar" with you so you can avoid this. better yet go BEFORE you strip down. BUT IF YOU HAVE TO GO go. you WILL stay warmer with a empty tank

A friend of my parents went on a Himalaya expedition a long time ago. During the night, his friend went on a bathroom trip in the storm. His friend was never seen again.
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gerritNov 2 '12 at 20:52

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@gerrit, a storm in Himalaya is a very extreme example;) I've made a lot of <0 C bathroom trips, some of them barefoot on the snow, and it's not that awful. Never tried during a heavy storm, though.
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SteedNov 15 '12 at 11:33

Two considerations: Layers should be loose and non-constricting so as to allow good circulation. Too many layers can get tight. Also, day clothes will be damp, even if you think they aren't. Air them before bringing them into your bag if it is cold.

I generally sleep naked in my sleeping bag. Ive slept nights where I went to sleep in my clothes, and then woke up because my feet were freezing in the middle of the night, so I took off my clothes and when back to bed, and then woke up at dawn toasty warm. And nights where I didn't do that in the same exact conditions, and suffered the night.

And these are dry clean clothes kept away from any moisture.

My theory is that my core produces a lot of heat, but my extremities are always cold. Sleeping naked lets the heat from my core make the whole sleeping bag warmer so my extremities can warm up. Layering up only reflects heat from your body back at that same part of my body, so I end up with an especially warm core, but freezing feet.

Only in contrived or extreme examples does wearing less clothing about your body in fact make you warmer when camping.

The areas where I might consider it warmer to not wear clothing inside a sleeping bag are:

Insufficient ground insulation when sleeping on solid ice or where you have no other viable insulation. In this case, it might make more sense to place the clothes between you and the bag or between the bag and the ground / tarp. This is especially true when the uncompressed side of your sleeping bag is sufficiently lofted but the compressed side is not.

Clothes that are wet and will chill you as the water evaporates or interfere with down insulation in the sleeping bag.

The obvious case where being naked results in additional heat generation due to companionship or "calisthenics" - especially if your clothes are too plaid for your party.

I've been in many high altitude expeditions in three Continents and have explored many vertical and horizontal caves and underground systems. Sleeping bags are best when they're good. Don't try to get a cheap offer and trade it for your safety or comfort.

In mountaineering, especially when camping at very high altitudes (5,000 metres or higher) it is recommended to sleep naked.

First, we use only down feathers sleeping bags OR linear single fiber ones -Hollofill is one of the materials made that way. The specs can go from good for 0ºC to -40ºC. Kelty, Big Agnes, Valandre, Marmot... there are many options with high quality standards. Differences are huge in price between a good sleeping bag and a bag for laying down.

One of the main reasons why sleeping naked is more comfortable and safe is that moisture in our clothes stays there for a much longer period of time than on the skin.

Second, the air cushion of warm air that our body produces by means of its own heat, is maintained between the skin and the inner layer of down -or fibers- and these let the moisture pass through their loose structures, then it's evaporated or frozen on the outside shell of the sleeping bag, keeping your body dry.

One should always make sure to undress when already inside the bag. The boots and all layers of socks must be kept inside the sleeping bag as well. This helps to dry them and keep them warm for the next morning. You can leave your parka and windbreakers outside and use them as pillows.

Going to the loo in such situation will take its time. But, if weather is just around -10ºC, you can grab your thermic undies and go outside without much fuss. Just make it fast!

Camping at warm places asks for another layer, if possible: use a light cotton sheet inside the sleeping bag. It will pass the sweat to the outside as vapour and you'll feel that smooth and fresh sensation when in contact with cotton.

Never cover a proper sleeping bag with anything. This will shut the shell's properties of heat exchange and you'll feel colder or smothered.

Try sleeping naked at home. Always take a shower before bedtime and you'll experience why sleeping naked is much better. Keep your pj's under the covers and use them if you have to get out of bed. You'll find yourself rested and fresh in the morning.

There is more than one reason, which makes you feel warmer sleeping with less clothes (even if it's perfectly dry):

It's the same deal as with mittens, which are warmer than gloves. When you wear a lot of clothes, there is additional separation between the parts of your body and more exposed surface. More surface means more heat transfer from the body to the environment. And if you share a bag with someone, you get less heat from one another.

What's more, your arms and legs, which produce less heat, get separated by thick clothes from your torso (and from each other). So you end up with uncomfortably (or even painfully) cold toes or arms.

In winter/high altitude you are usually wearing a lot of clothes and it's quite bulky. When you get into your sleeping bag like this, some parts of your body get compressed/clamped (e.g. armpits, elbows, ...). This leads to worse blood circulation and less heat for peripherial parts of your body.

There is some balance, actually: you should wear something, but not too much. I personally prefer sleeping completely naked, but high up or in winter I use Polartec-100 linen, socks and a cap with a scarf (though in these conditions I'm usually sharing the bag).

And all this doesn't mean you have to put your clothes in your backpack. You can use it to insulate your most exposed parts: the one which are closest to the tent and to the earth (actually snow). Just don't put it on but keep it loose between yourself and the sleeping bag, effectively improving bag's thickness.

From my experience - no way. If the temperature outside is below the comfort range of your sleeping bag, you'll have to put some extra things on you, otherwise you risk hypothermia (in best case - sleepless night). The best things are the fleece ones.

However, when it goes to trousers and jackets (normal or rain-proof, without warming), it's usually a better idea to put them under your sleeping bag or foam-pad, to increase isolation from ground (the most head you loose through ground). So actually it may be a source of that hypothesis - better to sleep without trousers, and sleep on them instead.

However, many people claim that it's warmer to sleep naked. Well, the ones I know have very warm (and expensive) sleeping bags, so the temperatures were above their comfort rage. I have an impression that such rumors are produced by sleeping bag producers, which want people to buy warmer sleeping bags for cold weather, instead of trying to keep them warm in their (cheaper) summer-time sleeping bags with extra layers of clothing.

Note that sleeping bags are relatively new. As a soldier in 1st World War, you had only a blanket to keep you warm and you had to sleep in everything you have on you during winter.

Person I slept with less layers while in the military and sometimes naked and was warmer than I ever was when I slept with layers on. I don't really know if it's dependent upon the type of bag you're in or what, but I have 0 doubt that it worked much better for me personally.

Keep the bag Warm not your clothes!

Both clothes and sleeping bags are insulators, the problem with wearing too many clothes in your sleeping bag is that you will actually insulate yourself independent of the bag, as a result, your bag won't get warm or it will take a long time to warm up. You will eventually have issues with moisture control, as your bag will slowly warm up after you fall asleep as heat escapes your layers and eventually replaces the cold air in the loft of the bag, causing you to sweat. There's nothing worse than waking up with that clammy sleeping bag feel.

The best solution:

Layer your bag! – Cotton or silk bag liners will actually add close to an extra 10°C to your bag, in addition, adding a vapour barrier will draw the moisture away from your bag and condense it on the liner, keeping your bag dryer and warmer by a few more degrees. Not only will you stay warmer, but you will also be way more comfortable. In really cold weather you can even consider layering with a second sleeping bag.

There are multiple benefits to layering your bag:

You can stay warmer with fewer layers of clothing on

Less moisture condensation in the bag keeps your bag dryer and therefor warmer

Better heat distribution prevents cold spots/toes

More comfort in bag means a much better nights sleep

You protect your bag the same way sheets protect a mattress, keeping your bag cleaner

Less time needed to air out your bag/let it dry in the mornings

The best routine: strip down naked and put on a DRY base layer before getting into your bag. I usually sleep with nothing more than a polyester base layer and a pair of wool socks in cold weather (except for a touque and neck-warmer to keep the drafts at bay). In the morning, I change out of my sleeping layer back into my hiking clothes.

This is of course assuming that you already have a good ground sheet and sleeping pad, which are essential components of a warmer sleep system.